1. Field of the Invention
The present invention relates to a battery system with a current detection circuit capable of extremely reliable current detection, and in particular relates to a battery system with a current detection circuit used as a power source apparatus in a vehicle such as a hybrid car or electric automobile (electric vehicle) optimally suited for stably detecting battery charging and discharging current.
2. Description of the Related Art
In a power source apparatus used in a vehicle such as a hybrid car, high reliability and high precision detection of battery charging and discharging current is in demand. This is because the vehicle cannot be driven by the batteries when the battery current cannot be detected. A state-of-the-art current detection circuit uses a difference amplifier to amplify the voltage across a current detection resistor in series with the batteries, and detects current from the output voltage of the difference amplifier. Current can be detected from the difference amplifier output voltage because the voltage across the current detection resistor is proportional to the current. To reduce power loss due to the voltage drop across the current detection resistor, the resistance of the current detection resistor is made extremely small. Since the voltage across the current detection resistor is proportional to the product of the resistance and the current, a small resistance means the voltage is also small. The small resistor voltage is amplified by the difference amplifier to detect the current.
This current detection circuit cannot determine if there is an open-circuit in the current detection lines that connect the current detection resistor to the difference amplifier inputs. This is because difference amplifier input voltage is zero when a current detection line is open-circuited, and difference amplifier input voltage is also zero when no current is flowing. Consequently, the absence of current flow cannot be distinguished from an open-circuited current detection line.
To resolve the drawback described above, the present applicant developed a battery system that can detect current detection line open-circuit. (Refer to Japanese Laid-Open Patent Publication 2009-139223.)
In the battery system of JP 2009-139223-A, current detection line open-circuit can be detected, but two high precision difference amplifiers are required. Accordingly, this system has the drawback that the circuit structure complexity and parts-cost increases.
The present invention was developed with the object of further correcting the drawbacks described above. Thus, it is a primary object of the present invention to provide a battery system with a current detection circuit that can detect current detection line open-circuit and can reliably and stably detect battery current with a simple circuit structure.